Wind turbines are used to convert wind energy to electrical energy in a clean and efficient way. In a wind turbine a rotor comprising rotor blades drives an electric generator, either directly or by means of a gearbox. The alternating current (AC) frequency that is developed at the stator terminals of the generator is directly proportional to the speed of rotation of the rotor. The voltage at the stator terminals also varies as a function of the rotational speed of the generator. For an optimum energy capture, this rotational speed varies according to the speed of the wind driving the rotor blades. To limit the energy capture at high wind speeds and to avoid a damage of the rotor, the rotational speed of the generator is controlled by altering the pitch angle of the rotor blades.
An adaptation of the variable voltage and frequency of the electric generator to a nominally fixed voltage and frequency of a power grid is typically achieved by a power converter. A power converter typically includes a generator bridge, which in normal operation operates as an active rectifier to supply power to a direct current (DC) link. The generator bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy. A power converter typically comprises two network bridges, wherein a first network bridge converts the AC power signal provided by the generator to a DC power signal and a second network bridge converts this DC power signal to an AC power signal, which in voltage, frequency and phase angle is matched to the power grid.
WO 2010/018424 A1 discloses a method of controlling a power converter to deliver an amount of active power and an amount of reactive power to a three-phase power grid. The method comprises providing a wind-powered multi-phase generator and an AC-AC converter operating in a PWM mode. The AC-AC converter has a set of converter input terminals connected to the three-phase generator and a set of converter output terminals connected via a converter impedance to a set of grid input terminals of the three-phase grid. The method further includes providing a control unit comprising a measurement unit for measuring current and voltage and a microcontroller running a control algorithm for generating a current reference value. The measurement unit measures the current and/or the voltage on the converter output terminals and/or on the power grid input terminals. The AC-AC converter regulates the current on the grid input terminals such that it corresponds to the current reference value.
EP 1 995 863 A2 discloses a method of controlling a plurality of power converters that can be used to interface to a supply network or a power grid. Each power converter includes a network bridge operating in accordance with a PWM strategy, which has the same switching period and which causes at least one unwanted harmonic in the voltage of the power grid. The method includes the step of providing the switching period of the PWM strategy of each network bridge with a different time offset relative to a time datum such that the at least one unwanted harmonic in the supply network voltage is at least partially cancelled.
A wind farm or wind park, also known as wind power plant, is a collection of a few tens or a few hundreds of wind turbines installed in close vicinity with respect to each other. Within a wind farm the electric power generated by the various wind turbines is aggregated at a common collector bus (bus bar), which hereinafter is also denominated a Point of Common Coupling (PCC).
Wind farms are usually located in rural places several miles away from a power grid. Therefore, an electric power transmission link must be used in order to connect the PCC with a substation of a power grid. This often results in a “weak” grid at the PCC. At the PCC disturbances in the power grid e.g. caused by asymmetric loads of the power grid will be seen as an unbalance where, in an electric vector diagram showing the phase angles and the magnitudes of each phase, the phase offset and the magnitudes aren't equal during the disturbance. For a balanced system the magnitude of the voltage vectors are equal to each other and the phase offsets between the different phases are 120°. For an unbalanced system the magnitudes aren't equal nor are the phase offsets.
In power engineering it is a common practice to treat an unbalanced system by a superposition of symmetrical components. This means that the unbalanced system is composed into a positive sequence, a negative sequence and a zero sequence. The positive sequence represents a vector diagram of a balanced system, wherein the vectors rotate in counterclockwise direction. The negative sequence represents a vector diagram of a balanced system, wherein the vectors rotate also in counterclockwise direction. The zero sequence represents just a single vector rotating also in the counterclockwise direction.
For a balanced system there exists only a positive sequence. The negative sequence will show up only in case of some unbalance. An existence of the zero sequence is an indication for a fault in the electric network.
For a typical wind farm application the positive sequence vectors may have an amplitude of e.g. 132 kV. The negative sequence vectors typically have an amplitude of e.g. 150 V to 200 V. Of course, the ideal case would be if the negative sequence vectors have an amplitude of 0 V, but that is very unlikely in reality.
There may be a need for balancing an electric system comprising a wind farm and a power grid in order to get a better voltage quality.